There is described a device for stabilization of a wellhead where the upper portion of a wellhead casing projects up above a seabed, the upper wellhead casing portion being sideways supported in a suction substructure, more particularly in that a blowout preventer valve projecting up from the upper wellhead casing portion, is provided with multiple support elements abutting supportingly against an edge portion of an end cover on the suction substructure. Also described is a method for stabilizing a wellhead, an upper portion of a wellhead casing projecting up above a seabed and being sideways supported in a suction substructure surrounding the wellhead casing portion and extending downward in an uncompacted material. Finally the use of a suction substructure for support of a wellhead is described.
Installation of elements on a wellhead, particularly a blowout preventer valve (BOP), on top of a wellhead casing extending down through uncompacted materials in the seabed, generally with an upper portion of the wellhead casing surrounded by and fastened in a conductor casing, normally involves a risk for fatigue of the wellhead casing in that sideways forces are applied to the wellhead so that the wellhead casing is being bent. The side loading may occur as a result of drift of a riser extending up through the mass of water from the wellhead and up to a surface installation. When a blow out preventer weighs 250-500 tons and has a vertical extent of up to 14-16 meters and a horizontal extent of 5-6 meters, such a bending strain will increase by the load resting on the wellhead casing having its centre of gravity displaced away from the original vertical central axis of the wellhead. The problem is described inter alia by Dahl Lien: “Methods to Improve Subsea Wellhead Fatigue Life”, a project assignment at the Faculty for engineering science and technology, the Institute for petroleum technology and applied geophysics, NTNU, Trondheim 2009. The situation may lead to deformation of the wellhead casing and at worst fatigue failure.
From prior art describing solutions to the problem of fatigue of the wellhead casing forming a substructure for wellhead elements, the present inventors own suction substructure (Conductor Anchor Node=CAN), described in NO patent 313340, may be mentioned, in its entirety included here by reference, and which in principle provides a larger contact surface between the upper part of the conductor casing and the surrounding seabed mass, the diameter of the suction substructure typically being 8 meters, while the conductor casing diameter is in the range 0.75-0.90 m (30-36 inches).
In NO 328221 the “Vasshella Flexible Casing Joint” (VFC) is described as a device for a combined wellhead and pipe string arranged to be set down in an outer casing being installed in a well bore in the seabed, an articulated joint being introduced between the casing string and the wellhead, the joint being able to transfer axially acting forces at the same time as it cancels bending moments on the wellhead casing.
It is also known (Dahl Lien 2009) to use moorings extending aslant outward and downward from an upper portion of a wellhead installation to the seabed where the moorings are fastened to anchors.
From NO 305179 is known a suction anchor surrounding an upper portion of a conductor casing and parts of a wellhead. To the wellhead is connected a frame arranged to carry a swivel device for horizontal connection of a riser etc., and the frame rests on separate suction anchors placed at a distance from the first mentioned suction anchor.
In the further description the term “wellhead valve” comprises both a blowout preventer (BOP) alone and also a combination of a blowout preventer and other types of valves (for example production valves), and other types of valves or valve type combinations alone, as said wellhead valve is arranged on a wellhead on an end portion of a wellhead casing projecting up above a seabed.